A Josephson contact is a weak link between two superconductors, in which the two Josephson effects are to be observed. This weak link can be a superconducting region, the transition temperature of which is lower than the transition temperature of the adjacent superconductor. The weak link can also be a normal conductor. Finally, the weak link can also be an insulating layer which is thin enough for electrons to "tunnel" through it. Therefore, the "sandwich" of a so called Josephson tunnel contact consists of two superconducting layers which are separated from each other by a thin insulating layer.
For example, the superconducting layers are formed from metallic strips, 1 mm wide and 2000 .ANG. thick, which arc successively vapor-deposited on a glass plate (see Scientific American, vol. 214, 1966, pages 30 through 39). Tin, for example, may be used as a superconducting metal. Between the two tin strips, having a thickness of about 2000 .ANG., there is a very thin insulating layer having a thickness of about 10 .ANG., which has been formed by oxidation of the tin to form tin oxide. If the two tin strips are equipped with electrical contacts for current and voltage and the whole arrangement is cooled below the transition temperature for tin in a Dewar vessel, then the physical conditions are given under which Josephson effects can be observed. Because of the low transition temperature for tin (less than 4 Kelvin) this Josephson contact is cooled with pumped liquid helium. In said Dewar vessel the inner vessel, which is cooled with liquid helium is in addition surrounded by an outer vessel, which is cooled with liquid nitrogen.
A Josephson contact cannot only be constructed from super conductors like tin, lead or niobium, but also from ceramic superconductors, which have been known since 1986. While the transition temperature of the classical superconductors, which were discovered in 1911, usually lies between 5 and 20 Kelvin and thus only barely above absolute zero, ceramic superconductors can already lose their electrical resistance at temperatures which are essentially higher. In 1990 the highest known transition temperature was around 125 Kelvin (Spektrum der Wissenschaft, October, 1990, pages 118 through 126). The ceramic superconductors are therefore also called high temperature superconductors. For those high temperature superconductors which have a transition temperature above 77 Kelvin, it is sufficient to use liquid nitrogen for cooling in order to achieve a superconducting state. Liquid nitrogen cooling is simple and can be done at a low price.
For ceramic superconductors one has to deal with crystals, in which different metal oxides form a complicated crystal structure of different coordination polyhedra. A coordination polyhedron is defined as an energetically stable spatial arrangement of large metal atoms, surrounded by small oxygen atoms. It is the chemical bond between copper and oxygen, which is crucial for electrical behavior of almost all high temperature superconductors. In addition to copper further metal atoms like lanthanum, barium, calcium, bismuth, strontium etc. are deposited in the crystal compound. The coordination polyhedra of copper oxide are arranged in planes or double planes.
Within the framework of the present application a crystal is defined as both a single-crystal, which is drawn from a melt, and also a crystal layer, which is deposited epitaxially on a substrate.
In order to fabricate Josephson contacts for ceramic superconductors, a grain boundary between two adjacent crystals is used as a weak spot. From the reference Superconductor Industry mentioned at the beginning, it can be concluded that grain boundary contacts of this nature do not perform as well as Josephson contacts formed of classical low temperature superconductors. For fabricating the thin films of the boundary grain contacts, the methods of laser deposition and photolithography are resorted to. With the laser deposition, for example, three successive layers grow epitaxially on a substrate. Two main problems of these grain boundary contacts are named. The exact location on which the contact between the superconducting and non-superconducting layers occurs is random and is barely controllable. Besides, the generation of a larger number of contacts is not yet controllable.
The principal construction of a Josephson contact is shown graphically in FIG. 1a. The cross symbol between the two superconducting layers stands for the weak link through which a supercurrent I flows as a result of the DC-Josephson effect. The voltage drop at the Josephson contact, which is labelled U, is equal to zero and the supercurrent I flows without any conduction loss when the conditions for the DC-Josephson effect are present. Under the conditions of the AC-Josephson effect the voltage drop U becomes finite so that power is transduced in the contact.
The Josephson effects are assumed to be known. A detailed description is given for example in the already mentioned article Scientific American, vol. 214, 1966, pages 30 through 39, which will be referred to. Accordingly, the electron pairs, so-called Cooper pairs, can tunnel across an insulating barrier between two superconducting regions due to their wave nature. Very many Cooper pairs exist in each superconductor. These Cooper pairs occupy a macroscopic quantum state with the wave function .PSI.=const e.sup.i.phi., whereby .phi. is called the phase of the wave function.
FIG. 1b shows the effect of this concept on a Josephson contact. At a cross section through the succession of layers the superconducting charge carriers are not distributed uniformly. In the two superconducting regions there is a high density of Cooper pairs, while only few or no Cooper pairs are present in the weak spot. The Cooper pair density in the two superconductors SL1 and SL2 can be of the same magnitude, but does not have to be. In the given superconductor SL1 the wave function has the phase position .phi..sub.1, in the superconductor SL2 the wave function has the phase .phi..sub.2. If the superconductor SL2 was far away, the Cooper pairs in the two separate superconductors would have two determined, but independent phase conditions .phi..sub.1 and .phi..sub.2. Through the Josephson contact according to FIG. 1a, the phases of the two regions are coupled. Josephson has shown, that the supercurrent I, which flows through the contact according to FIG. 1a as a powerless DC-current, follows the simple relationship EQU I=I.sub.c sin.gamma. (1) EQU .gamma.=.phi..sub.1 -.phi..sub.2,
where .gamma. is the phase difference and I.sub.c the maximum possible DC-supercurrent.
While, with the DC-Josephson effect, the whole region behaves like a single superconducting region in spite of the weak link, the AC-Josephson effect can be observed under physical conditions, exhibiting a finite potential drop U at the contact. If the transport current I, which is forced into the contact, exceeds the maximum supercurrent I.sub.c, the voltage drop U at the contact leads to a change of the phase difference .gamma. with time. According to Josephson, the following relationship applies: ##EQU1##
This effect will be assumed to be known. At a specific voltage drop U the phase difference .gamma. increases continuously. Because of the sinusoidal relationship (1), the Josephson current I oscillates with frequency, which obeys the following relationship: EQU .gamma.=(2e/h)U (2a)
This relationship is called Josephson voltage-frequency-relationship. The high frequency AC-current in the contact is related to the radiation of an electromagnetic field. The frequency lies in the microwave region.
The most common model for describing a Josephson contact is the equivalent circuit according to FIG. 2. The DC-Josephson effect occurs, when the externally applied current I is small enough to overcome the barrier (cross symbol) by means of superconduction. With higher transport at current I, a voltage drop U occurs, which not only causes the charging of a capacitor C of the contact and an additional DC-current through a resulting resistance R of the contact, but also causes the supercurrent I.sub.c x sin .tau. to oscillate according to equation (2). In view of the AC-current, the current branch with the cross symbol corresponds to an inductance, so that the AC equivalent circuit resembles a damped oscillating circuit. From the equivalent circuit the following differential equation can be derived, relating the external DC-current I to the time-varying phase difference .gamma.: ##EQU2##
Besides the external current source an external microwave field can also act on the Josephson contact. In this case the equivalent circuit has to be extended by an AC-current source (FIG. 3). Accordingly the differential equation is extended by the term I.sub.Ac sin .omega.t. The external microwave frequency interferes with the internal Josephson current frequency, so that current jumps occur at determined voltages at the current voltage characteristic of the contact (Shapiro effect).
Classical superconductors, typically consisting of the metals lead, niobium and tin including certain alloys of these metals, have already found a plurality of applications. Belonging to them, a closed superconducting loop with two Josephson contacts, which has become known by the name SQUID (Super Conducting Quantum Interference Device). This deals with the most sensitive contemporary device for detecting magnetic fields. In medical applications, for example, the weak magnetic fields of the brain currents are measured.
A further application follows from the transition from the DC-current Josephson effect to the AC-current Josephson effect. When the critical current I.sub.c is exceeded, the finite voltage drop occurs instantly. The abrupt occurrence of the voltage U is the fastest and least dissipative switching process which is currently known. Based on this ultrafast switches for digital technology have been constructed.
With the AC-current Josephson effect the frequency of the radiated electromagnetic wave is proportional to the voltage drop at the contact. Thus, continuously tunable high-frequency emitters can be built. The highest frequencies achievable to reach up into the THz-range.
An especially important application is the Josephson voltage normal. This application is based on impinging high-frequency fields, i.e. on the alternate circuit according to FIG. 3. During tile operation of such contacts current steps occur in the current voltage characteristic at determined Josephson voltages (Shapiro effect). According to the voltage-frequency relationship (2a) the accuracy of these Josephson voltages depends only on the accuracies with which the microwave frequency and the nature constants contained in the equation can be given. These uncertainties are very minute; the Josephson ratio 2e/h=4.8359767 only has a relative uncertainty of 0.3 per million. The inverse of the Josephson ratio lies around 2.068 .mu.V/GHz. Since 1990 the Federal Republic of Germany has also been using relationship (2a) to define the units of electrical voltage. In order to obtain voltages in the order of 1 Volt for calibrating secondary voltage normals, the German National Bureau of Standards (Physikalisch Technische Bundesanstalt) is currently employing series connections of several thousand Josephson contacts at frequencies of around 90 Ghz.
It is important that, with all mentioned applications, liquid helium has to be used for cooling. In addition, the fabrication of the Josephson contact, on the basis of classical superconductors, is expensive.
With high temperature superconductors which are attractive because of the low amount of cooling required, it is much more difficult to fabricate Josephson contacts. Therefore, applications of high temperature superconductors as Josephson contacts have so far only been realized as single contacts for SQUID's.